1. Field of the Invention
The present invention relates to a transflective-type liquid crystal display device.
2. Related Background Art
Along with the development of infrastructure such as wireless or radio transmission, in the field of small-size portable equipment, in addition to the conventional so-called “standalone equipment”, there have been demanded some functions such as functions as network connection equipment including Internet connection, and functions as high-quality and high-image quality portable or mobile terminal equipment, for example, for receiving television signals. Particularly, by virtue of simplicity in the communication performance and portability, portable phones (cellular phones) have expanded their applications from applications of conventional mere portable telephones to applications in many fields in daily life such as terminals for settlement in merchandise purchase and terminals for a person to go through the automatic ticket gate in railroads.
A significant increase in utilization time has led to a necessity for portable terminals, which have begun to be used in such extensive applications, to stand up for significant issues. Such issues are a requirement for high image quality and a problem of power consumption. Display of television images is also possible from the viewpoint of infrastructure of communication, but on the other hand, the necessary number of display pixels is much larger, and the brightness is higher, as compared with conventional displays for cellular phones. Further, while high definition display and high brightness display are required, in the case of portable terminals of which the premise is battery operation, a higher level of power saving is required.
In order to satisfy the above contradictory function requirements and to utilize advanced communication infrastructure which is being established, a novel display system which can simultaneously satisfy high image quality and power saving should be rapidly established.
Attempts to satisfy such requirements have already been made, and a certain stage of function can be provided. This is a liquid crystal display (LCD) known as the so-called “transflective” or “semi-transmissive display.”
In the case of the portable terminals which are in many cases used outdoors, in an environment with strong external light, when ambient light is utilized for use as reflective display, power consumption in the backlight for the display can be saved. Particularly, in the high image quality display LCDs, there is a tendency that the utilization efficiency of backlight generally decreases with increasing the number of pixels, and display which does not use backlight has great significance for power saving.
On the other hand, in the use of LCDs during evening hours or in the indoor use, it is also necessary to impart satisfactory brightness to the display by using the backlight. In the transflective-type LCDs, optimal image quality can always be selected depending upon illuminance environment used in this way, and, at the same time, power consumption can be reduced to a small value. Therefore, the transflective-type LCD can be the to be an ideal display as a display device for high-performance portable terminals. In this connection, in the case of a light-emitting display, very bright display can be provided under dark illuminance environment. On the other hand, under environment of illuminance like outdoor environment of fine weather, the brightness becomes relatively insufficient. In order to provide a higher level of brightness, for example, an additional function is necessary wherein illuminance of environment is detected and the brightness is automatically enhanced.
Transflective LCDs can provide ideal display as the above display for high-performance portable terminals, but on the other hand, they have the following problems remaining unsolved.
That is, in the same display panel, transflective-type LCDs, as indicated by its name, should be used as transmissive LCD or reflective LCD depending upon illuminance of environment. In general, LCD is of the so-called “birefringence control type” which controls the polarization of light. Accordingly, in the determination of optical characteristics, the retardation, that is, the product (Δn×d) of the anisotropy of refractive index of a liquid crystal material as an optical medium (Δn) and the optical path length of light which is incident to LCD and finally exits from LCD (d) should be a given value. In general, in a conventional liquid crystal display system represented by TN, Δn is specified as a material constant of a liquid crystal material. Therefore, the setting of the retardation is mainly designed by the optical path length, that is, by the cell gap of a liquid crystal display panel. The optical design of LCD is most fundamentally conducted so that, for a center wavelength 560 nm (green) of visible light, the value of Δn×d is π/2. For example, when the Δn value of the liquid crystal material used is 0.07, in order to maximize the utilization efficiency of light (backlight or external environment light), the value of “d” satisfying (0.07×d)/0.56 μm (560 nm)=1/2, that is, 4 μm, should be selected.
Herein, care should be taken to a difference in the optical path length between transmissive LCD and reflective LCD. FIGS. 1(a) and FIG. 1(b) schematically show the optical path length of the transmissive LCD and the optical path length of the reflective LCD, respectively. As can be seen from FIGS. 1(a) and FIG. 1(b), the optical path length of the reflective LCD is twice that of the transmissive LCD. That is, it is found that, in the transmissive LCD, light emitted from backlight is passed through the liquid crystal material layer only once, whereas, in the reflective LCD, light introduced from the outside is passed through the liquid crystal material layer twice to provide an optical path length of Δn×2d.
A so-called “one pixel-two cell gap” system has been devised as a method of overcoming the problem of the optical path length in the transflective-type LCD. FIG. 2 is a schematic view showing the one pixel-two cell gap system. As shown in FIG. 2, the retardation for both transmission and reflection is satisfied by a method wherein one pixel is divided into at least two parts and the cell gap in one pixel is set to “d” for transmission while the cell gap in the other pixel is set to d/2. This method is very effective for effectively satisfying both the transmissive display and the reflective display. On the other hand, however, two cell gaps should be built in one pixel. This poses a very serious problem in the production of LCDs for portable terminals where high-definition high-density display is generally required. Particularly, as described above, in the high-definition display is required to provide a high-image quality display such as TV image display in the future, a further reduction in pixel size and an increase in the resolution are necessary. This further makes it difficult to produce such satisfactory LCDs.
Thus, in such a situation that the development of a small-size high-definition display is required more than ever before, not only a high-image quality display and a low-power consumption display, but also a high-performance high-definition display with satisfactorily high productivity is demanded.